Pullet

How can you tell if it's a hen or rooster. All I get from my Friends is crazy little old time ways that just really get the best of me. I purchase 3 little ones and 3 straight ones. But then I went back and the people are not educated like me about the kind or anything else. But the lsat trip they had pullets and others so I was wondering how the could tell. I raised little one a 28 years ago and it was so much fun for me I enjoy that kind of stuff. Do the chicks need medicine in the water? I'm not sure why I thought this but if anyone could let me know I would appreciate the information. Starting over with this is not like riding a bike... Being I want hens should I go and purchase they now knowing they are hen? Would that be a wise thing???